Motion vector estimation in television images

ABSTRACT

Apparatus for motion vector estimation in a television image uses a block matching technique with successive refinement of the motion vector estimate. The apparatus comprises a vector filter and a vector calculator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following applications for U.S.Letters Patent, each of which was filed concurrently herewith, that is,on May 31, 1988, and has a common assignee herewith:

Ser. No. 07/199,680

Ser. No. 07/199,681

Ser. No. 07/199,682

Ser. No. 07/199,683

Ser. No. 07/200,421

Ser. No. 07/200,503

Ser. No. 07/200,531.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to motion vector estimation in television images.Such motion vector estimation is particularly, but not exclusively, usedin television standards converters and in slow motion processors.

2. Description of the Prior Art

International television program exchange necessitates standardsconverters due to the different television standards used in differentcountries, for example, the 625-line 50-fields per second (625/50) PALsystem used in the U.K., and the 525-line 60-fields per second (525/60)NTSC system used in the U.S.A.

Many different standards converters have been previously proposed. Oneof the best known is the ACE (Advanced Conversion Equipment) developedby the British Broadcasting Corporation. Basically ACE operates on aninput digital television signal line-by-line to derive interpolatedsamples required to form an output digital television signal.Interpolation is done not only spatially using four successivehorizontal scan lines of the input television signal, but alsotemporally using four successive fields of the input television signal.Thus, each line of the output television signal is derived bymultiplying respective samples from sixteen lines of the inputtelevision signal by respective weighting coefficients.

Further details of ACE will be found in U.K. patent specification No.GB-A-2 059 712 and in `Four-field digital standards converter for theeighties` by R. N. Robinson and G. J. Cooper at Pages 11 to 13 of`Television` (the journal of the Royal Television Society) forJanuary/February 1982.

Although ACE gives good results, there is the problem that the equipmentis very bulky. To overcome this problem, we have previously proposed atelevision standards converter comprising three field stores and four4-line stores for receiving an input digital television signal of onestandard and deriving therefrom arrays of sixteen lines, each arrayconsisting of four successive lines from each of four successive fieldsof the input television signal. A weighting coefficient store storessets of sixteen weighting coefficients, respective sets corresponding topositions both spatial and temporal of respective lines of an outputdigital television signal of a different standard, relative to thesixteen lines of the input television signal. Two interpolation filtersthen derive line by-line the output television signal by multiplyingcorresponding sample values from each of the sixteen lines of the inputtelevision signal by a respective weighting coefficient in a set ofweighting coefficients and sum the resulting products to form aninterpolated sample value, and four output field stores receive andstore the derived lines of the output television signal. To store theadditional lines which are derived when the output television signal hasmore lines than the input television signal, a 45-line store isinterposed between one of the interpolation filters and the output fieldstores. Further details will be found in our U.K. patent specificationNo. GB-A-2 140 644.

The performance of such standards converters which employvertical/temporal interpolation techniques represents a compromisebetween generating blurred pictures while maintaining good motionportrayal and maintaining vertical resolution but at the expense of`judder`. The former is a result of post filtering in order to preventdisturbing alias effects; the latter is a result of the intrusion of theadjacent 2-dimensional repeat sample structures.

We have therefore proposed that motion vector estimation should beincorporated in television standards converters and in slow motionprocessors. The problem with the majority of existing motion vectorestimation methods is that their use is biased towards video conferencetype applications where generally the subject matter is either a singleperson's head and shoulders or a small group of people seated around atable. With television images of this type the motion is relativelysimple in comparison with broadcast television images where for exampleat a horse race meeting the camera could be following the leaders in arace. In this situation the motion would be complex, for example,because the camera would be panning. Thus, the background may well bemoving at speeds greater than eight pixels per field, while in theforeground there would be at least one horse galloping. This means thatthe motion vector estimation method must try to track the horses legs,which may well be moving in different directions to that of the alreadymoving background.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an improved method ofmotion vector estimation in a television image.

Another object of the present invention is to provide an improvedapparatus for motion vector estimation in a television image.

Another object of the present invention is to provide an improvedtelevision standards converter.

Another object of the present invention is to provide an improved slowmotion processor.

According to the present invention there is provided a method of motionvector estimation in a television image using a block matching techniquewith successive refinement of the motion vector estimate.

According to the present invention there is also provided apparatus formotion vector estimation in a television image using a block matchingtechnique with successive refinement of the motion vector estimate, theapparatus comprising a vector filter and a vector calculator.

The above, and other objects, features and advantages of this inventionwill be apparent from the following detailed description of illustrativeembodiments which is to be read in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in very simplified block diagrammatic form a televisionstandards converter;

FIG. 2 shows in block diagrammatic form a first television standardsconverter;

FIG. 3 shows in block diagrammatic form a second television standardsconverter;

FIG. 4 shows part of the standards converter of FIG. 3 in more detailedblock diagrammatic form;

FIG. 5 shows time charts for explaining the operation of FIG. 4;

FIG. 6 shows part of the standards converter of FIG. 2 in more detailedblock diagrammatic form;

FIG. 7 shows time charts for explaining the operation of FIG. 6;

FIG. 8 shows part of the standards converter of FIG. 2 in more detailedblock diagrammatic form;

FIG. 9 shows time charts for explaining the operation of FIG. 8;

FIG. 10 shows part of the standards converter of FIG. 2 in more detailedblock diagrammatic form;

FIG. 11 shows part of the standards converter of FIG. 2 in more detailedblock diagrammatic form;

FIG. 12 shows part of FIG. 11 is more detailed block diagrammatic form;

FIG. 13 shows a timing chart for explaining the operation of FIG. 12;

FIG. 14 shows part of the standards converter of FIG. 2 in more detailedblock diagrammatic form;

FIG. 15 shows a flow chart for explaining the operation of FIG. 14;

FIG. 16 shows part of FIG. 14 in more detailed block diagrammatic form;and

FIG. 17 shows part of the standards converter of FIG. 2 in more detailedblock diagrammatic form.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order more readily to understand the motion vector estimation whichforms the subject of the present invention, the form and operation oftwo standards converters and a slow motion processor which use suchmotion vector estimation will first be described. The standardsconverters to be described maintain vertical resolution and remove the`judder` by compensating for motion between fields. In effect the motionbetween consecutive fields is analyzed. These fields can then be`aligned` pixel by pixel such that they represent static pictures uponwhich conversion can then take place. As a result, vertical resolutioncan be maintained.

The standards converters to be described can be divided into two parts.The first part is analogous to a known standards converter performingvertical/temporal interpolation to convert between 525/60 and 625/50television standards. Alone, this would generate an output in whichvertical resolution would be maintained but with the added effect ofjudder. To remove this judder four fields of the input digitaltelevision signal which are used in the conversion process are alignedunder the control of motion vectors generated from a motion analyzerwhich forms the second part of the standards converter.

This is shown in very simplified diagrammatic block form in FIG. 1. Thevideo portion of an input digital television signal of one standard,which may for example have been derived by sampling an analog televisionsignal at 13.5 MHz, is supplied to an interpolator 1 from which thevideo portion of the required output television signal of a differentstandard is derived. A motion analyzer 2 receives the luminance videoand derives motion vectors which provide data representing the motionbetween successive fields of the input television signal to control theoperation of the interpolator 1. The interpolator 1 operates in agenerally similar manner to the corresponding portion of a knownstandards converter, for example as referred to above. It also, however,contains the means to align the four fields used in the interpolation,under the control of the motion vectors.

The repositioning of the four fields is performed in two stages. Thefirst stage involves varying the address of a variable delay elementassociated with each field to reposition the picture to the nearest lineor sample. The second stage uses interpolation techniques bothvertically and horizontally to reposition to within ±1/16 line or ±1/8of a sample. Even with no movement, both the above techniques are usedto enable conversion of line standards.

The vertical interpolator has four taps per field allowing effectivelyan 8-tap vertical filter to be applied to the static pictures. An 8-tapinterpolator allows good vertical resolution to be maintained withminimal distortion. The effect of distortion in the horizontalinterpolator is less of a problem, so a 2-tap horizontal filter is used,although a 4-tap horizontal filter, for example, may be used.

The temporal interpolator is used in normal operation to enableinterpolation of perspective changes or when no sensible motion vectorcan be detected, in which case the interpolator 1 must revert to normalstandards conversion operation where no picture re-positioning occurs.

When converting from a high field rate to a lower rate, the incomingfields are interpolated such that an interpolated field can occasionallybe dropped without any movement deterioration. All the interpolation isdone at the input field rate and passed to a time base corrector whichthen spreads the fields generated over the required time period for theoutput standard.

The above operation is necessary when converting from 525/60 to 625/50.It is also evident however that 625 lines must be generated where only525 lines exist in the input signal.

To overcome the line number conversion problem a second time basecorrector is used at the input to produce a signal having 585 lines atthe 60 Hz rate. A 585-line format can contain all the active pictureinformation in the 625-line format. Following this first time basecorrector there are occasional lines which have no video information.The interpolator stores are frozen during this time, so that anadditional interpolated line can be generated from the same lines usedto generate the previous output line. This process allows 625 lines tobe interpolated from the original 525.

The reason for selecting the 585/60 format will now be explained in moredetail. A 625-line picture contains 288 active lines in each field, and720 samples in each horizontal line at the sampling rate of 13.5 MHz.The circuits, to be described below, of the television standardsconverters of FIGS. 2 and 3 use techniques which allow the picture to beshifted horizontally by plus or minus twenty-four samples. This requiresa minimum horizontal blanking of forty-eight samples. The total numberof sample positions required in a field is therefore:

    (720+48)×288=221184.

There are clearly considerable advantages in using a 13.5 MHz clockthroughout the system, in which case the number of clock cycles within a60 Hz period (more exactly a 59.94 Hz period) is:

    225225.

If 576 lines of data are required in one frame, the number of horizontalsamples would be 782.03125. Although this number is sufficient tocontain the required (720+48) samples, the fractional sample would meanthat the structure was non-orthogonal on a line to line basis. Thiswould cause significant design difficulties in the rest of the standardsconverter, so the number of lines required was gradually increased, from576, until a whole number of samples, in fact 770, existed in each line.

The only format that achieves the orthogonal structure is the 585/60format, which in addition gives a useful vertical blanking of four linesin the first field, five lines in the second field and fifty samples ofhorizontal blanking.

In the 625/50 to 625/50 slow motion mode referred to below there is norequirement to store the active video of the 625 format within a 60 Hzperiod, so the interpolation and other processing is done in the normal625/50 format.

When converting from a low field rate to a higher rate the input timebase corrector is required to produce a video stream at the output rate.This is done by occasionally repeating an input field. When the repeatedfield occurs, all the interpolator stores must be frozen so that theinterpolation is applied to the same input fields used to create theprevious output field.

If this technique were not used, two sets of interpolator and movementdetector would be required to make up the missing field.

The above operation is performed when converting from 625/50 to 525/60.To allow 625 lines to exist during a 60-fields per second period againrequires the 585/60 intermediate format to be adopted. During thisprocess some of the interpolated lines will not be required, as only 525have to be produced from the original 625. A time base converter istherefore required on the output to produce the final 525/60 format.

The amount of interpolation required is determined by comparing inputand output synchronization pulse phases.

As mentioned above, motion analysis is performed on the luminance of theinput video. The method employed involves a number of stages to arriveat a single motion vector for each pixel. Movement can be detected inthe range ±24 pixels horizontally and ±8 (field rate) vertically.

In a first stage, motion in the picture at points on the screen spacedsixteen samples horizontally and eight lines vertically is determinedusing a block matching technique. The original motion vectors in a fieldare calculated every sixteenth sample and every eighth line. Each one ofthese points is at the centre of a search block. Conceptually each blockis scanned ±24 samples horizontally, and +8 and -8 samples verticallyover the next field each time generating the summation of thedifferences between the two fields over the area of the search block.The minimum overall difference then indicates in which direction theobject at that point has moved.

In practice, the above technique is applied in separate steps whichgreatly reduces the amount and complexity of hardware required:

Step 1

Test for minimum difference in just three positions, centre position,sixteen samples to the left, sixteen samples to the right.

Step 2: Starting from point indicated above.

Test for minimum difference in nine positions symmetrically distributedabout the above starting point in steps of eight samples or lines.

Step 3: Starting from point indicated above.

Test for minimum difference in nine positions symmetrically distributedabout the above starting point in steps of four samples or lines.

Step 4: Starting from point indicated above.

Test for minimum difference in nine positions symmetrically distributedabout the above starting point in steps of two samples or lines.

Step 5: Starting from point indicated above.

Test for minimum difference in nine positions symmetrically distributedabout the above starting point in steps of one sample or line.

Step 6

After step 5, the motion of the object has been detected to the nearestpixel. A more accurate vector value can be achieved by adding a sixthstep in which the difference produced at the final position indicated bystep 5 is compared with the two differences above and below to adjustthe vertical vector value and with the two differences to the left andright to adjust the horizontal vector value.

The above technique relies on achieving correlation between thereference search block and a similar block of video data on thefollowing field (the search positions). In step 5 it is possible thetrue movement was a half pixel more or less than detected, but it isnecessary for the best correlation to occur at this point, even althoughexact correlation cannot be achieved. To ensure this occurs, the picturecan be filtered both vertically and horizontally by a gaussian filterwhich has +6 dB attenuation at 1/2 Nyquist frequency.

Similarly, for step 4, the picture can be filtered with a 6 dBattenuation at 1/4 Nyquist frequency, which allows a one pixel error indetection.

Step 3 uses a picture filtered with a 6 dB attenuation at 1/8 Nyquistfrequency allowing a two pixel error.

Step 2 uses a picture filtered with a 6 dB attenuation at 1/16 Nyquistfrequency allowing a four pixel error.

Finally, step 1 uses a picture filtered with 6 dB attenuation at 1/32Nyquist frequency allowing an eight pixel error. In addition, becausethe pictures are so heavily filtered during steps 1, 2, 3 and 4, thesamples can be reduced, for example halved in number, which stillfurther greatly reduces the number of calculations and amount ofhardware required.

The effective search block size is sixteen lines high and forty-eightsamples long. A large search block is necessary to detect accurately themovement of large plain areas. The central part of plain areas areunimportant as the values of the pixels at these points do not changefrom one field to the next, but the edges of such objects are obviouslyimportant. If the detection of motion is limited to ±24 sampleshorizontally and ±8 lines vertically then a block of the above sizewould be the minimum size to ensure accurate motion detection.

In the standards converters, depending upon the conversion modes, theluminance video entering the motion analyzer 2 is in various forms of585-lines/60-fields per second. This might comprise repeated lines for525 input or repeated fields for 625 input. In addition, the inputcontains both field polarities. The first process is to ensure acontinuity of data and single field polarity for the motion estimationprocessing. This is done by interpolation on the input data by a vectorinterface to maintain continuity, and filtration horizontally to aidsubsequent motion detection/correlation.

Separate outputs from this circuit are passed to motion estimationvector filters and motion detection field stores/vector selectors. Theoutput of the vector interface is, as described above, spatiallycontinuous, single field polarity data. The output to the fieldstores/vector selectors depends upon the input and output modes. In somemodes it is continuous, and in others it contains repeated lines/fields.The vector filters and vector calculators perform the steps outlinedabove.

The processing of the various steps is performed by vector calculatorsand a vector processor. The vector calculators perform steps 1 to 5 andthe vector processor performs step 6. In addition, the vector processorperforms the second stage in the motion estimation, as follows:

For each 8×16 block a choice is made of four from seven motion vectors,the seven motion vectors being the one for that particular block and thesix for the six nearest blocks respectively.

In addition, the vector processor also determines the four most commonmotion vectors throughout the whole input field, these being calledmodal motion vectors. The primary use of the modal motion vectors is inthe border areas close to the edge of a field where it is not possibleactually to calculate any local motion vectors. Also, if any one or moreof the local motion vectors are equal, then these are substituted for bythe modal motion vectors.

In the next stage of motion detection, for each pixel, the four motionvectors are tested by producing the difference between the extrapolatedpositions on field 0 to field 1. During standards conversion a fieldneeds to be interpolated between two fields; say between field 0 andfield 1. So the motion vectors generated between these two fields areconsidered to be most representative of the motion. Four motion vectorsare used from these two fields. To decide which is the correct motionvector a pixel from field 0 is compared with a pixel from field 1 usingthe motion vector to decide where the pixel to be generated had comefrom on field 0 and where it has gone to by field 1. Mathematically, ifthe position x, y, z must be generated, where; x=horizonal position,y=vertical position, z=temporal position between field 0 and field 1,the pixels used for comparison are as shown below. Field 0 is assumed tobe at z=0 and field 1 at z=1. ##EQU1##

For each motion vector a modulus of the difference between the pixelsindicated in field 0 and field 1 is found. The minimum difference isassumed, as a first estimate, to indicate the correct motion vector. Ifa number of motion vectors produce a very similar difference then thesemotion vectors are tested again using a comparison between fields -1 and0.

    Pixels from field -1

    x.sup.-1 =x-(1+z)V.sub.h

    y.sup.-1 =y-(1+z)V.sub.v

The minimum modulus of difference of the remaining vectors produced bythis second test is then considered to represent most accurately themotion vector.

If a number of motion vectors again have similar differences then anoption exists to assume no movement. If only the horizontal componentvaried and the vertical component did not, then only the horizontalcomponent would be set to zero and the vertical component would bemaintained at the detected value. If only the vertical component varied,then the horizontal component would be maintained and only the verticalcomponent set to zero. If the pixel difference chosen is too large thenan option exists to set the whole motion vector to zero in bothdirections.

A final stage is applied once every pixel has been assigned a motionvector. Here the motion of each pixel is tracked from one field to thenext and a recursive filter applied to the motion vector value. Thisremoves the effects of noise and small movement estimation errors andalso smooths the trajectory of the motion vectors.

There are two possible ways of tracking the motion of a pixel.

In the first, the motion vector for a pixel in field t is used to pointto a pixel in field (t+1). The motion vector determined for this pixelin field (t+1) is then recursively filtered to form the final motionvector for the pixel in field (t+1).

In the second, the motion vector for a given pixel in field t is used topoint to a pixel in field (t-1). The motion vector from this pixel isthen recursively filtered with the motion vector for the given pixel toform the final motion vector for this given pixel in field t.

In either case the final output is a motion vector for each pixel whichis passed from the motion analyzer 2 to the interpolator 1 to beemployed in aligning the four fields used in the standards conversionprocess.

The first standards converter for converting an input digital 625-line50-fields per second television signal to an output digital 525-line60-fields per second television signal is shown in detailed block formin FIG. 2.

The incoming video at 50-fields per second and a sample rate of 13.5MHz, that is CCIR 601 data, is supplied to a demultiplexer 31 whichseparates it into luminance components Y, synchronizing signals SYNC andchrominance components UV. The luminance components Y are supplied to a4-field luminance time-base corrector (TBC) 11Y and the chrominancecomponents UV are supplied to a 4-field chrominance TBC 11C. Thesynchronizing signals SYNC are supplied to a control 32 together with aninput field polarity signal from an external input, and an output fieldsynchronizing reference signal from another external input. The TBCs 11Yand 11C occasionally repeat fields, so that the output is at 60-fieldsper second. The control to the TBCs 1Y and 11C which causes therepetition of a field is derived from the input field synchronizationpulses, and the required output field synchronization pulses. Thecomparison of the synchronization pulses also provides a temporal offsetfigure which indicates the amount of temporal interpolation required atthe outputs of the TBCs 11Y and 11C such that smooth motion at 60-fieldsper second would be observed.

When converting from 50-fields per second to 60-fields in this way aline conversion of 625 to 525 is necessary. It is therefore necessary tomaintain the original 625 lines of information at a 60-fields per secondrate so that they are all available to form the interpolated lines.

The standards converter uses an intermediate standard which is capableof containing all the active vertical information of the 50-fields persecond signal at the 60-fields per second rate. The intermediatestandard also contains all the active line information arrangedorthogonally line by line still using the original 13.5 MHz sample rate.

The intermediate standard used, and which is as explained above capableof meeting all these requirements, is a 585-line format at 60-fields persecond. When sampled at 13.5 MHz each line of this format has exactly770 samples. It is clear therefore that 585 lines is sufficient tocontain the 576 active lines of the 625-line format at a 60-fields persecond rate. As the active line width is only 720 samples there is stillfifty samples of horizontal blanking.

The luminance data (D) from the luminance TBC 11Y is supplied by way ofa processing compensating delay 17Y to a luminance temporal shiftregister 16Y comprising four field stores (FS) 12Y, 13Y, 14Y and 15Y.The luminance TBC 11Y also supplies a temporal freeze signal (F) by wayof the delay 17Y to the shift register 16Y. The chrominance TBC 11Csupplies the chrominance data (D) by way of a processing compensatingdelay 17C to a chrominance temporal shift register 16C which comprisesfour field stores 12C, 13C, 14C and 15C. The chrominance TBC 11C alsosupplies a temporal freeze signal by way of the delay 17C to the shiftregister 16C. Associated with the shift register 16Y is a luminanceinterpolator 1Y which receives inputs from each of the field stores 12Y,13Y, 14Y and 15Y, and derives the 585-line format. The output of theluminance interpolator 1Y is supplied to a 2-field luminance TBC 18Y.Associated with the shift register 16C is a chrominance interpolator 1Cwhich receives inputs from each of the field stores 12C, 13C, 14C and15C, and also derives the 585-line format. The output of the chrominanceinterpolator 1C is supplied to a 2-field chrominance TBC 18C. When theoutputs of the TBCs 11Y and 11C are frozen, during a repeat field, theshift registers 16Y and 16C are also frozen, so that four distinctconsecutive fields of the input always exist in the shift registers 16Yand 16C. Thus the shift registers 16Y and 16C are used to provide thetemporal taps for the interpolators 1Y and 1C.

Each temporal tap produces four line taps at a position depending on themotion vectors, so that a 2-dimensional filter can be used to providethe necessary interpolation. The interpolated picture will contain 576active lines, so that a correct picture will be obtained when everysixth line in one field is dropped. The 484 lines left produce theactive picture portion of the 525-line format. To enable lines to bedropped in this way, the outputs from the interpolators 1Y and 1C aresupplied to the 2-field TBC 18. The TBCs 18Y and 18C write in all 576/2lines, but only read out the required 484/2 lines to provide therequired output television signal. The outputs of the luminance TBC 18Yand of the chrominance TBC 18C are supplied to a multiplexer 34 whichmultiplexes the luminance components Y and the chrominance components UVto provide output CCIR 601 data in the form of a digital 525-line60-fields per second television signal.

The control 32 supplies control signals (C) to the luminance TBC 11Y andto the chrominance TBC 11C. The control 32 also supplies control signalsto the luminance TBC 18Y and the chrominance TBC 18C. It also suppliesinterpolation control signals (IC) to the luminance interpolator 1L andthe chrominance interpolator 1C.

The luminance data only, as supplied by the luminance TBC 11Y, is alsosupplied to the motion analyzer 2 shown in the upper part of FIG. 2, sothat motion vectors can be generated. In fact a frame delay is necessarybetween the TBCs 11Y and 11C and the shift registers 16Y and 16C toallow for the time taken to process the motion vectors. The freezing ofthe shift registers 16Y and 16C must therefore also be delayed by oneframe, and these delays are provided by the delays 17Y and 17C.

The motion analyzer 2 comprises a vector interface 35 to which theluminance data from the luminance TBC 11Y is supplied, together with theinterpolation control signal from the control 32. The vector interface35 supplies data interpolated to 625 lines to a vector filter 36 and avector calculator 37 which together perform the motion estimationdescribed above. The output of the vector calculator 37 is supplied to amodal motion vector processor 38 and also to a sub-pixel motionestimator 39. The motion vector processor 38 supplies four outputs andthe sub-pixel motion estimator one output to a motion vector reducer 40which supplies four outputs to a vector selector 41.

The vector interface 35 also supplies the data interpolated to evenfields to a processing compensating delay 42 to which it also suppliesthe received interpolation control signal, and also a temporal freezesignal (F) generated at the vector interface 35. The data from the delay42 is supplied to a temporal shift register 43 which comprises threefield stores 44, 45 and 46 which supply respective data outputs to thevector selector 41. The delay 42 supplies the interpolation controlsignal to the vector selector 41 which supplies the selected motionvector to a recursive motion vector filter 47, the output of which isthe vector data which is supplied to the luminance interpolator 1Y andto the chrominance interpolator 1C.

The way in which the motion analyzer 2 derives the vector data has beendescribed in detail above, and will be further described below, but theoperation of the elements 35 to 43 and 47 will now be briefly described.

The vector interface 35 receives the luminance data from the luminanceTBC 11Y, and the interpolation control signals from the control 32. Itsupplies 625-line data, normally contained within the 585/60 format, tothe vector filter 36. It also supplies data to the delay 42. These datamust contain a picture which is the same line standard as the requiredoutput, again normally contained within the 585/60 format. Each field ofthe interpolated data is also made to appear even.

The vector filter 36 produces the filtered picture data required forsteps 1 to 5 above of the motion detection. The filtered picture dataare supplied in sample reduced form to the vector calculator 37.

The vector calculator 37 operates on the filtered and sample-reduceddata from the vector filter 36 using an algorithm described in the termsof the steps 1 to 5 above of the motion detection. The process isessentially a two-dimensional binary search for motion down topixel/line resolution. For each field, 1200 motion vectors are generatedand are supplied to both the modal vector processor 38 and to thesub-pixel motion estimator 39. It also supplies surrounding weightedabsolute difference (WAD) values as calculated by step 5 above to thesub-pixel motion estimator 39. For details of WAD calculations, see`Advances in Picture Coding`, Musmann et al, Proceedings of the IEEE,April 1985. The specific WAD value which is the minimum in step 5 aboveof the motion detection provides a figure of merit (FOM).

The vector processor 38 calculates the four most common motion vectorsthat are detected in each field and supplies them to the vector reducer40.

The sub-pixel motion estimator 39 receives the motion vectors from thevector calculator 37 together with the surrounding WAD values. Fromthese it estimates sub-pixel movement to be appended to the motionvector values. With each motion vector its corresponding final WAD valueis also supplied to the vector reducer 40.

The vector reducer 40 receives the motion vectors from the vectorprocessor 38 and from the sub-pixel motion estimator 39. For each motionvector from the sub-pixel motion estimator 39, the six motion vectorsclosest to it are grouped together. For each motion vector there arethen eleven choices. The reduction process selects four motion vectorsfrom the eleven for supply to the vector selector 41.

The vector reducer 40 supplies the vector selector 41 with fourrepresentative motion vectors for each sixteen pixel by eight line blockof the picture. By comparing pixels over three fields, the vectorselector 41 selects the single best motion vector for each pixel in thepicture. The motion vector selected is supplied to the motion vectorfilter 47.

The delay 42 delays the data by one frame less twenty-one lines tocompensate for other delays in the system.

The temporal shift register 43 holds and supplies the three fields ofdata used by the vector selector 41.

The motion vector filter 47 tracks a motion vector from one field toanother so applying some filtering to the motion vectors by combiningmotion vectors in different fields, so reducing motion detection errors.The output of the motion vector filter 47 is supplied to the luminanceand chrominance interpolators 1Y and 1C to control the alignment of thefield data.

Exactly the same hardware can be used as a slow motion processor withgood motion portrayal for either a 625/50 or a 525/60 television signal.It is not however necessary to use the vertical interpolator to providethe line number conversion. In all cases the control 32 determines whataction is required by recognizing the input/output standard from theinput and output field synchronization pulses. In slow motion the inputfield polarity is used.

Whereas in 50-fields per second to 60-fields per second conversion afield was occasionally repeated, in slow motion the field is repeatedthe same number of times as the input field is repeated. As repeatedfields are not written into the shift registers 16Y and 16C, the shiftregisters 16Y and 16C again contain distinct consecutive fields. Indeedif a video tape recorder reproduces without any interpolation of itsown, the original interlace structure is maintained allowing fullresolution pictures to be produced. The temporal offset required iscalculated by comparing the actual field rate pulses, be they 50-fieldsper second or 60-fields per second, with the rate at which a new fieldis received. To determine the temporal offset in this way, the systemneeds a signal to be available which indicates the true field polarityof the field being repeatedly replayed. The vertical interpolator willalways produce the field polarity required at the output.

Conceptually the TBCs 11Y and 11C are not really required for slowmotion operation, but their presence does provide a framesynchronization facility and also simplifies the system configuration.

The second standards converter for converting an input digital 525-line60-fields per second television signal to an output digital 625-line50-fields per second television signal is shown in detailed block formin FIG. 3.

In this case, interpolation requires that all the input data isavailable in a consecutive form. In this case it would not therefore bepossible to convert to 50-fields per second before the interpolators 1Yand 1C. The input data however contains only 484 active lines and theinterplators 1Y and 1C must produce 576. The 2-field TBCs 18Y and 18Care therefore positioned at the front of the standards converter toprovide the necessary time slots for 484-line to 576-line conversion.

The original continuous line structure is written into the TBCs 18Y and18C but is read out in the 585-line standard with approximately everysixth line being blank. The interpolators 1Y and 1C are then used toproduce a continuous picture at the output line rate by freezing itsline stores during the blank input line, and producing the requiredadditional line at the output, so ensuring that a spatially correctpicture is produced. The required temporal offset is detected andapplied as in the first standards converter, although the interpolationis applied such that a field can occasionally be dropped leaving themotion smooth. The field is dropped such that 60-fields per second to50-fields per second conversion is achieved. The dropping of a field isachieved by using the 4-field TBCs 11Y and 11C at the output.

Thus the second standards converter differs from the first standardsconverter shown in FIG. 2 in only minor respects. In particular, theluminance TBCs 11Y and 18Y are interchanged, and the chrominance TBCs11C and 18C are also interchanged. Also, no temporal freeze signals arerequired.

In both cases the control 32 has various functions as follows;controlling the reading and writing of the TBCs 11Y, 11C, 18Y and 18C;generating a temporal offset number, and in the case of the firststandards converter the temporal freeze signal, and generating avertical offset number together with vertical interpolation controlsignals. These functions will now be described in more detail.

Firstly, the 2-field luminance and chrominance TBCs 18Y and 18C alwaysswitch between field stores at the end of every 60 Hz field. However,the operation of the 4-field luminance and chrominance TBCs 11Y and 11Cdepend on the mode of operation, and their control is also associatedwith the generation of the temporal offset signal. In fact, the controlof the luminance and chrominance TBCs 11Y and 11C is determined from theinput and output field synchronizing signals.

The derivation of the temporal offset signal in the case of 525/60 to625/50 operation will now be described with reference to FIGS. 4 and 5.

In FIG. 4, the control 32 is shown as including a line counter 61, andfirst and second latches 62 and 63. A line clock signal is supplied to aclock terminal of the line counter 61, while the input fieldsynchronizing signal is supplied to a reset terminal of the line counter61 and to a clock terminal of the second latch 62. The output fieldsynchronization signal is supplied to a clock terminal of the firstlatch 62. The output of the line counter 61 is supplied to the input ofthe first latch 62, the output of which is supplied to the input of thesecond latch 63, the output of which is the temporal offset signalsupplied to the luminance and chrominance shift registers 11Y, 11C, 18Yand 18C.

The input and output field synchronizing signals are shown in FIGS. 5Aand 5B respectively. FIG. 5C shows the output of the line counter 61which repetitively counts from 0 to 524. FIGS. 5D and 5E show theoutputs of the first and second latches 62 and 63 respectively. Bylatching the counter 61, the required proportion of the input fieldperiod is determined. The temporal shift value tn indicates the positionbetween two input fields where the output field must be interpolatedsuch that when the shaded field shown in FIG. 5A is dropped, continuousmotion still occurs. Thus, the field which uses the temporal offsetshown shaded in FIG. 5E is the one that is dropped. It will be seen byreference to FIGS. 5A and 5B, that the field which is dropped is the onewhich does not have a new temporal shift associated with it. The field(arrowed) which is to be dropped is indicated to the following circuitryby the temporal freeze signal.

The derivation of the temporal offset signal in the case of 625/50 to525/60 operation will now be described with reference to FIGS. 6 and 7.

In FIG. 6, the control 32 is shown as including a line counter 71 and alatch 72. A line clock signal is supplied to a clock terminal of theline counter 71, while the input field synchronizing signal is suppliedto a reset terminal of the line counter 71. The output fieldsynchronization signal is supplied to a clock terminal of the latch 72.The output of the line counter 71 is supplied to the input of the latch72, the output of which is the temporal offset signal supplied to theluminance and chrominance shift registers 11Y, 11C, 18Y and 18C.

The input and output field synchronizing signals are shown in FIGS. 7Aand 7B respectively. FIG. 7C shows the output of the line counter 71which repetitively counts from 0 to 624. FIG. 7D shows the output of thelatch 72. By latching the counter 71, the required proportion of theinput field period is determined. Thus, the temporal shift value tnagain indicates the position between two input fields where the outputfield must be interpolated, such that if the shaded field is repeated,continuous motion still occurs. The field which is repeated is the onewhich has two temporal shift values associated with it. The field(arrowed) which is to be repeated is indicated to the followingcircuitry by the freeze signal.

The deviation of the temporal offset signal in the case of slow motionwhether at 525/60 to 525/60 or 625/50 to 625/50 is the same, and willnow be described with reference to FIGS. 8 and 9.

In FIG. 8, the control 32 is shown as including a line counter 81, afield counter 82, first to fourth latches 83 to 86, an exclusive-OR gate87 and a scaler 88. The input field synchronizing signal is supplied toa clock terminal of the first latch 83, to a clock enable terminal ofthe field counter 82, and to a second reset terminal of the line counter81. The input field polarity signal is supplied to the first latch 83and thence to the second latch 84 and also to one input of the gate 87.The second latch 84 supplies an output to the second input of the gate87, the output of which is supplied to a first reset terminal of theline counter 81, to a reset terminal of the field counter 82 and to aclock terminal of the third latch 85, which forms a speed detectorlatch. A line clock signal is supplied to a clock terminal of the secondlatch 84, and to respective clock terminals of the line counter 81 andthe field counter 84. The output of the line counter 81 is supplied toan input terminal of the scaler 88, and the output of the field counter82 is supplied to an input of the third latch 85 and also to an offsetinput terminal of the scaler 88. The output field synchronizing signalis supplied to a clock terminal of the fourth latch 86. The output ofthe third latch 85 is supplied to a scale factor terminal of the scaler88, the output of which is supplied to the fourth latch 86, the outputof which is the temporal offset signal.

The input field synchronizing signal and the input field polarity signalare shown in FIGS. 9A and 9B respectively. FIG. 9C also indicates theinput field synchronizing signals and FIG. 9D the output fieldsynchronizing signals. FIGS. 9E and 9F indicate the operations of thefield counter 82 and the line counter 81, which are respectivelycounting fields and lines from 0 to N. FIG. 9G indicates the output ofthe fourth latch 86 which is the temporal offset signal. FIG. 9Hindicates the temporal freeze signal (which is active when low), and, asindicated by the arrows, the shaded field that uses the temporal offsetshown is a repeat of the previous field that used the temporal offsett1.

To generate the temporal freeze signal, the control 32 is shown in FIG.10 as including a synchronous RS flip-flop 91, a latch 92, an inverter93 and an AND-gate 94. The output field synchronizing signal is suppliedto one input of the flip-flop 91, to the input of the inverter 93 and toa clock enable terminal of the latch 92. The input field synchronizingsignal is supplied to the other input of the flip-flop 91, while a lineclock signal is supplied to clock terminals of the flip-flop 91 and thelatch 92. The output of the flip-flop 91 is supplied to one input of thegate 94, which receives at its other input the output of the inverter93. The output of the gate 94 is supplied to the input of the latch 92,the output of which forms the temporal freeze signal. The operation ofthis circuit is such that if more than one output field synchronizingpulse follows an input field synchronizing pulse, a freeze occurs.

Referring back to FIG. 2, the generation of the vertical offset numberby the control 32 will now be described. The same address generatorwhich reads data from the luminance TBC 11Y into the luminanceinterpolator 1Y and the motion analyzer 2, also addresses an erasableprogrammable read-only memory (EPROM) which provides the vertical offsetnumber together with vertical freeze signals when required.

(In the FIG. 3 arrangement which is used for 525/60 to 625/50, the readaddresses of the luminance TBC 18Y are used, but in all other modes theread addresses of the luminance TBC 11Y are used.)

The vertical offset number is generated assuming that both the input andthe output fields are even, and it then indicates the position betweentwo input lines where the output line must be interpolated such that anon-distorted picture would be produced if: a line were occasionallydropped in 625/50 to 525/60 conversion, or a line were occasionallyrepeated in 525/60 to 625/50 conversion.

When a line is repeated by the luminance TBC 11Y (18Y), a verticalfreeze signal is generated.

If the input fields are not both even, then the interpolators 1Y and 1Cmust make use of the input field polarity and output field polarity toensure correct interpolation.

The contents of the EPROM are generated in a way similar to thatdescribed above in connection with FIG. 10 for the temporal offsetsignal, using the known line position in both a 525 and a 625 picture.

The form and operation of the vector filter 36 and the vector calculator37, with which the present invention is particularly concerned, will nowbe described in more detail with reference to FIGS. 11 to 17.

Referring to FIG. 11, which shows the vector filter 36 in block form, itcomprises step 1 to step 5 stores 101 to 105, two selectors 106 and 107,a filter store 108, a finite impulse response (FIR) filter 109 and asample reducer 110 connected as shown. The steps 1 to 5 are thosereferred to above.

The vector filter 36 receives as its input the data comprisingconsecutive 625-line fields of the same polarity (in this case even)from the vector interface 35 (FIGS. 2 or 3). As the fields are of thesame polarity they are all in the same spatial position. Inherent in theprevious process of aligning all fields to the same polarity is atwo-dimensional filtering function which means that the data supplied tothe vector filter 36 has been sufficiently filtered to be used as thestep 5 (or final step) data. This step 5 data is stored in the vectorfilter 36 while the remaining operation is carried out. This operationis recursive in nature under control of the selectors 106 and 107, inthat the previous step's data are filtered in two-dimensions and stored,then the process is repeated on that step data. Due to the amount offiltering involved it is possible to sample reduce the filtered outputin both dimensions after each step. Once this process has been carriedout four times, the vector filter 36 will then have available at itsoutputs as indicated the data for steps 1 to 5 of the vectorcalculation.

The FIR filter 109, the sample reducer 110 and the filter store 108perform the two-dimensional filtering sample reduction function. A moredetailed diagram of this area is shown in FIG. 12.

This part of the vector filter 36 comprises the filter store 108, theFIR filter 109 which is a 7-tap filter, registers (R) 121 and 122, asynchronously loadable register (SLR) 123, selectors 124 and 125, aninverter 126, and row and column address generators 127 and 128.

As can be seen from FIG. 12, the two-dimensional filter operation iscarried out by a single one-dimensional filter, the reason that this ispossible being that the desired two-dimensional response is actuallyvariable separable in form, meaning that a two dimensional convolutionin the time domain can be performed as a series of two one-dimensionalconvolutions. In this particular application, the vertical andhorizontal responses are identical and take the form of a cosine-squaredor raised cosine function, whose frequency response is such that thefilter output will be greater than 48 dB down at the Nyquist frequency,and 6 dB down at half the Nyquist frequency. This response isimplemented using the single 7-tap FIR filter 109 which is implementedusing two 4-tap FIR filter chips in cascade with an end filtercoefficient being set equal to zero. As the filter coefficients do notchange at all during operation they could be loaded into the filterchips on switch on, but in practice the circuit is designed such thatthe coefficients are reloaded every frame, as it is possible to programinto a programmable read-only memory (PROM) up to fifteen further setsof coefficients should the need arise, and these can then be selected bya 4-way bit switch mounted on the front of the board.

The operation of this circuit for the input of say step 5 data andproducing step 4 data as an output, this output having beentwo-dimensionally filtered and sample reduced is as follows.

The input in this instance is a normal field of data, and this will behorizontally filtered by the 7-tap FIR filter 109 operating at fullclock rate. The output of the FIR filter 109 is fed to the SLR 123 whichis controlled by a half clock rate signal, the effect of which is tomiss out every other sample thus reducing the filtered outputhorizontally by a factor of two.

Horizontally filtered and reduced data from the SLR 123 is written intothe filter store 108 row by row. Once this process has been performedover the whole field, the filter store 108 will contain a fieldconsisting of the correct number of lines, but which has been reduced insize horizontally by two. Incidentally, because the filter response issuch that its output is greater than 48 dB down at the Nyquistfrequency, then when the sample reduction takes place, there will be anabsence of any alias frequency components in the SLR output.

The row and column addresses to the filter store 108 are now switchedover such that the data is read out from the filter store 108 column bycolumn, and fed back into the FIR filter 109 and sample reducer (SLR123), the output of which is the original input data reduced by a factorof two in both the vertical and horizontal directions, which in thisinstance would be the step 4 data and as such is written into the step 4store.

The process is then repeated to obtain the step 3 data by reading thestep 4 store 104 (FIG. 11), and feeding this data into the FIR filter109 and eventually into the step 3 store 103 (FIG. 11), likewise toobtain the step 2 data from the step 3 data. Obtaining the step 1 datafrom the step 2 is slightly different in that the filtration and samplereduction process is only carried out horizontally. This is because thefirst step of the motion vector estimation process looks for horizontalmovement (of sixteen pixels) only, and the next step, step 2, thenrefines the estimate found in step 1 by looking for motion in steps ofeight pixels/lines from the position indicated by step 1.

In FIG. 11, the step 5 store 105 is a field store, the step 4 store 104is a half field store, the step 3 store 103 is a quarter field store,the step 2 store 102 is an eighth field store, and the step 1 store 101is a sixteenth field store. In practice, steps 1, 2 and 3 are all storedin the same random access memory (RAM), with steps 4 and 5 being storedin independent RAMs.

The complete process in determining the step 1 to 5 data takes two fieldperiods, and it is then necessary to hold this data for two furtherfield periods as it is required to be used as search block data on onefield search area data on the next. Thus, there is a necessaryrequirement for four such vector filter boards in the motion vectorestimation system, and this is illustrated by the simple timing diagramshown in FIG. 13. From FIG. 13 it can be seen that vector filters A andC can have their outputs multiplexed together, and the same holds forvector filters B and D, this means that the vector calculators will haveavailable to them on two separate inputs continuous search block/areadata.

Referring to FIG. 14, which shows the vector calculator 37 in blockform, it comprises a step 1 to 3 store 131, A and B step 4 stores 132and 133, A to D step 5 stores 134, 135, 136 and 137, two weightedabsolute difference (WAD) calculators 138 and 139, and an outputprocessor 140 connected as shown, and receiving inputs as shown.

An array of thirty vertical by forty horizontal motion vectors per fieldare calculated every field, and to enable this to occur in real timeusing a reasonable amount of hardware, it is necessary to have such tenvector calculator boards as depicted in FIG. 14. Using this arrangementmeans that the motion vectors are calculated in four blocks of thirtyvertically by ten horizontally.

The data from the vector filters 36 (FIG. 11) are provided as acontinuous stream of data for each of steps 1 to 3, step 4 and step 5,and it is left to each of the vector calculators 37 to determine whichdata it requires in the calculation of each step.

The simplified flow chart given in FIG. 15 shows the operation of thestep 1 to 3 store 131, the step 4 stores 132 and 133, and the first WADcalculator 138. Being the starting point, the step 1 data is always in afixed and known position, the search block will be one pixel the searcharea three horizontal pixels, with the resulting vector from thiscalculation being either zero or ±16 horizontal. The vertical componentof the vector is always zero. In order to determine where the searcharea is to be written into the next step store, the vertical andhorizontal addresses, which count to a maximum that the search areacould have moved to, determined by the previous step, are fed intovertical and horizontal PROMs along with the previous vertical andhorizontal motion vector components from the first WAD calculator 138.The PROMs are effectively mapping the output address to the step storesby combining the motion vector components with the input addresses.

With reference to FIG. 15, each time a step 4 displacement iscalculated, its value is passed to one of the four step 5 stores 134 to137 (FIG. 14) which are necessary in order continuously to calculate thefinal step 5 outputs.

The function of the first WAD calculator 138 is to calculate anddetermine which of the nine motion vectors is the minimum, and thencorrespondingly modify the vertical and horizontal motion vectorcomponents.

FIG. 16 shows the first WAD calculator 138 in more detail. It comprisesa modular subtractor 151, an accumulator 152, five registers 153, 154,155, 156 and 157, an SLR 158, a comparator 159, an AND-gate 160, and twoPROMs 161 and 162 connected and receiving inputs as shown.

The A and B inputs are the search block and search area data from thestep stores. The first subtractor 151 will calculate the absolutedifference between the single pixel values on its two inputs. The outputis one of two inputs to the accumulator 152, the other input being theaccumulator output delayed by one clock. Therefore, the function of theaccumulator 152 is to calculate the WAD values for the nine positions onthe first four steps. The nine WAD values are successively clocked, asthey are calculated, to the comparator 159 which determines the minimumWAD value for each step by enabling the SLR 158 when a minimum isdetected. At the start of each step the first WAD value is forced intothe SLR 158.

At the same time the `minimum detected` signal also clocks the register155 which will then hold the number of the minimum WAD calculation. Thisnumber is then fed to the PROM 161 at the end of each step, along withthe vertical and horizontal motion vector components from the previousstep in order to determine the resultant motion vector.

The second WAD calculator 139 (FIG. 14) is virtually identical to thefirst WAD calculator 138 (FIGS. 14 and 16) but it does not contain thecircuitry enclosed by the dotted line in FIG. 16, this function beingincorporated into the output processor (FIG. 17). Also the second WADcalculator 139 is only required to calculate step 5 whereas the firstWAD calculator 138 calculates steps 1, 2, 3 and 4, and for this reasonthere are a number of control lines which have been missed out of FIG.16 for simplicity.

FIG. 17 to which reference is now made shows the output processorreferred to above. It comprises six registers 171, 172, 173, 174, 175and 176, an inverter 177, a RAM 178, and three PROMs 179, 180 and 181connected and receiving inputs as shown.

The output processor of FIG. 17 not only modifies the motion vectordetermined by step 4 such that the motion vector is now calculated topixel resolution again dependent upon which of the nine WAD calculationswas a minimum, but it also stores this WAD value as the figure of merit(FOM) used by the motion vector reduction circuitry. In fact all WADvalues are stored in the RAM 178, as once the minimum has beendetermined it is then the decision of the output processor as to whichother four WAD values are necessary in order for the next circuit, whichis the vector processor 38 (FIGS. 2 and 3), to calculate any sub-pixelmovement. To this end it may be necessary for the second WAD calculator139 (FIG. 11) to determine up to two further WAD values as the sub-pixelprocessor requires the WAD values directly above, below, to the left andto the right of the minimum. Thus, if the minimum were at any other thanthe centre position this would be the case.

Finally, it is now possible to supply the motion vector, calculated topixel resolution, along with its FOM and four other WAD values necessaryfor determining any sub-pixel movement to the vector processor 38 andthe sub-pixel motion estimator 39 (FIGS. 2 and 3) for further processingas described above.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in the art without departing from the scope andspirit of the invention as defined by the appended claims.

We claim:
 1. A method of motion vector estimation in a television imagecomprising using a digitized signal for representing said image, andusing a block matching technique with successive refinement of themotion vector estimate and in which motion in said image at pointsspaced a predetermined number of samples horizontally and apredetermined number of samples vertically is determined by the blockmatching technique.
 2. A method according to claim 1 wherein said blockmatching technique comprises the steps of:testing for minimum differencein three positions, the centre position of a block, a predeterminednumber of samples to the left, and the same predetermined number ofsamples to the right; starting from the point indicated above, testingfor minimum difference in nine positions symmetrically distributed aboutthe above starting point in steps of a smaller predetermined number ofsamples or lines; starting from the point indicated above, testing forminimum difference in nine positions symmetrically distributed about theabove starting point in steps of a still smaller predetermined number ofsamples or lines; starting from the point indicated above, testing forminimum difference in nine positions symmetrically distributed about theabove starting point in steps of a still smaller predetermined number ofsamples or lines; and starting from the point indicated above, testingfor minimum difference in nine positions symmetrically distributed aboutthe above starting point in steps of one sample or line.
 3. A methodaccording to claim 2 comprising a further step, subsequent to the laststep of claim 2, of comparing the difference produced at the finalposition indicated by said last step with the two difference above andbelow to adjust the vertical vector value, and with the two differencesto the left and right to adjust the horizontal vector value. 4.Apparatus for motion vector estimation in a television image using ablock matching technique with successive refinement of the motion vectorestimate, the apparatus comprising a vector filter and a vectorcalculator.
 5. Apparatus according to claim 4 wherein said vectorcalculator is arranged to perform the following steps on a digitizedsignal representing said image and supplied to said vector calculator bysaid vector filter;testing for minimum difference in three positions,the centre position of a block, a predetermined number of samples to theleft, and the same predetermined number of samples to the right;starting from the point indicated above, testing for minimum differencein nine positions symmetrically distributed about the above startingpoint in steps of a smaller predetermined number of samples or lines;starting from the point indicated above, testing for minimum differencein nine positions symmetrically distributed about the above startingpoint in steps of a still smaller predetermined number of samples orlines; starting from the point indicated above, testing for minimumdifference in nine positions symmetrically distributed about the abovestarting point in steps of a still smaller predetermined number ofsamples or lines; and starting from the point indicated above, testingfor minimum difference in nine positions symmetrically distributed aboutthe above starting point in steps of one sample or line.
 6. Apparatusaccording to claim 5 wherein said vector calculator is arranged toperform a further step, subsequent to the last step of claim 5, ofcomparing the difference produced at the final position indicated bysaid last step with the two differences above and below to adjust thevertical vector value, and with the two differences to the left andright to adjust the horizontal vector value.
 7. A television standardsconverter comprising:means comprising apparatus according to claim 4 foranalyzing the motion between consecutive fields of an input televisionsignal of one television standard; means then to align said fields so aseffectively to represent static pictures; and means to effect conversionusing said static pictures to derive the required output televisionsignal of a different television standard.
 8. A television standardsconverter according to claim 7 wherein said means to align said fieldsoperates to vary the address of a variable delay element to repositioneach pixel of the picture to the nearest line or sample, and thenreposition each pixel of the picture both vertically and horizontally toa fraction of a line and a fraction of a sample respectively.
 9. Atelevision standards converter according to claim 8 wherein saidvertical repositioning to a fraction of a line is done by a verticalinterpolator with four taps per field, and said horizontal repositioningto a fraction of a sample is done by a horizontal filter having two orfour taps.
 10. A television standards converter according to claim 7wherein said means to effect conversion comprises an interpolator foreffecting vertical/temporal interpolation, and said input televisionsignal is supplied to said interpolator by way of a time base correctorwhich produces therefrom a 585-line 60-fields per second televisionsignal.
 11. A television standards converter according to claim 10wherein said input television signal is a 625-line 50-fields per secondsignal, said time base corrector is a 4-field time base corrector, andthe output of said time base corrector is supplied to said interpolatorby way of a 4-field shift register.
 12. A television standards converteraccording to claim 10 wherein said input television signal is a 525-line60-fields per second signal, said time base corrector is a 2-field timebase corrector, and the output of said time base corrector is suppliedto said interpolator by way of a 4-field shift register.
 13. A 625-line50-fields per second to 525-line 60-fields per second televisionstandards converter comprising:a 4-field time base corrector forreceiving an input 625-line 50-fields per second digital televisionsignal; a motion analyzer comprising apparatus according to claim 5connected to the output of said time base corrector for analyzing motionin said input television signal; a shift register also connected to theoutput of said time base corrector; an interpolator for deriving samplesof a required output 525-line 60-fields per second digital televisionsignal in dependence on samples derived from said shift register andpicture motion data derived by said motion analyzer; and a 2-field timebase corrector for assembling said derived samples to form said outputtelevision signal.
 14. A television standards converter according toclaim 13 wherein said 4-field time base corrector derives a 585-line60-fields per second television signal from said input television signalfor supply to said shift register.
 15. A time base corrector accordingto claim 14 wherein said motion analyzer derives motion vectors independence on the motion between corresponding pixels in consecutivefields of said input television signal, and supplies said motion vectorsto said interpolator so as effectively to align said pixels in saidconsecutive fields to represent static pictures.
 16. A 525-line60-fields per second to 625-line 50-fields per second televisionstandards converter comprising:a 2-field time base converter forreceiving an input 525-line 60-fields per second digital televisionsignal; a motion analyzer comprising apparatus according to claim 5connected to the output of said time base corrector for analyzing motionin said input television signal; a shift register also connected to theoutput of said time base corrector; an interpolator for deriving samplesof a required output 625-line 50-fields per second digital televisionsignal in dependence on samples derived from said shift register andpicture motion data derived by said motion analyzer; and a 4-field timebase corrector for assembling said derived samples to form said outputtelevision signal.
 17. A television standards converter according toclaim 16 wherein said 2-field time base corrector derives a 585-line60-fields per second television signal from said input television signalfor supply to said shift register.
 18. A time base corrector accordingto claim 17 wherein said motion analyzer derives motion vectors independence on the motion between corresponding pixels in consecutivefields of said input television signal, and supplies said motion vectorsto said interpolator so as effectively to align said pixels in saidconsecutive fields to represent static pictures.
 19. A slow motionprocessor comprising:an input circuit for receiving an input digitaltelevision signal; a motion analyzer comprising apparatus according toclaim 4 for analyzing motion in said input digital television signal; ashift register for holding successive different fields of said inputtelevision signal; an interpolator for deriving samples of a requiredslow motion output digital television signal in dependence on the degreeof slow motion, samples derived from said shift register, and picturemotion data derived by said motion analyzer; and a 2-field time basecorrector for assembling said derived samples to form said slow motionoutput television signal.
 20. A slow motion processor according to claim19 wherein said input circuit is a 4-field time base corrector.
 21. Aslow motion processor according to claim 20 wherein said 4-field timebase corrector derives a 585-line 60-fields per second television signalfrom said input television signal for supply to said shift register.